Electrical filter



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A TTORNE Y Unite States Patent O ELECTRICAL FILTER John M. De Bell, Jr., Passaic, N. J., assignor to Allen B.

Du Mont Laboratories, Inc., Clifton, N. J., a corporation of Delaware Application September 18, 1952, Serial No. 310,291

2 Claims. (Cl. S33-73) This invention relates to electrical wave filters and particularly to filters operating in the higher frequency bands and utilizing distributed parameters, such as wave-guides and tuned cavities.

In television systems, there is frequently a need to have electrical filters characterized by at, usually wide, pass bands with sharp or controlled cutoff characteristics. Such filters are found in television transmitters where they are employed to remove part of the lower side band energy of a television signal as required to conform with legal requirements of the Federal Communications Commission. One such filter circuit which is exceptionally economical of elements and yet which achieves the desired band pass characteristics is known as a lattice filter. This lter configuration, which is actually seldom used in low frequency filters because of the narrow tolerances within which the filter components must be fixed, is known to be equivalent in many respects to a waveguide arrangement known as the magic T. The magic T may take on any one of a number of configurations as shown in chapter 8 of Microwave Duplexers-volume 14 of the Radiation Laboratory series, by Smullin and Montgomery.

One of the problems in devising a filter for operation in the higher frequency bands stems from the difficulty of connecting elements together without introducing undesired reactances and impedances. For instance, in a filter to remove the lower side band of a television signal having a carrier frequency in the ultra-high frequency band around 700 mc., it is necessary to provide means for reducing the amplitude of the output signal at a very high rate with respect to frequency since even a 6 mc. television signal is a small percentage of the 700 mc. carrier. This is equivalent to saying that the cutoff slope of the filter must be exceedingly steep. By utilizing magic T arrangements it is possible to couple tuned cavities of sufiiciently high Q to achieve the desired steep cutoff characteristics and yet not have these cavities react upon each other since the coupling between them may be more readily controlled. Furthermore, in a symmetrical combination of magic T arrangements, such as will be disclosed hereinafter, it is possible to eliminate the reected signal which otherwise would be returned by an asymmetrical combination to the transmitter and improperly affect its operation.

It is one object of this invention to provide an improved high frequency electrical filter using distributed components.

Other objects are to provide a symmetrical filter in which no interference is obtained from the reflected wave and to provide a filter in which a preferential coupling is maintained between high Q tuned elements.

Still further objects will in part be obvious and in part appear hereinafter following the description in the specification together with the drawings in which:

Fig. 1 shows one arrangement of elements connected as a filter according to the invention.

Fig. 2 shows the frequency response of the circuit in Fig. 1.

Fig. 3 shows a complete symmetrical filter arrangement according to the invention.

Fig. 4 shows successive steps of tuning the circuit in Fig. 3.

In the circuit in Fig. l, a signal source 11 is connected by means of a transmission line 12 to an input terminal of'a hybrid ring 13. As is described in the text referred to above, the hybrid ring is one form of the magic T. The output terminal of the hybrid ring 13 is in turn connected by a transmission line 14 to a device 16 which may be an antenna, another filter or other circuit.

The ring 13 itself is composed of four arms of which arms 17 and 18 have the characteristic impedance-Zo, which is the same as the impedance of lines 12 and 14, while arms 19 and 21 have the characteristic impedance A tuned cavity 22 is connected to the junction of arms 18 and 19 by means of a transmission line 23 while a similar cavity 24 is connected by means of a line 26 to the junction of arms 18 and 2l.

In the operation of the circuit in Fig. l, the cavities 22 and 24 are tuned to the frequencies f, and f2 as shown in the response curve of Fig. 2. The attenuation sulfered by the signal from source llrin passing from the input terminal of ring 13 to the output terminal is indicated by the curve in Fig. 2. The slope of the cutoff characteristics of the circuit of Fig. l are indicated by the width of the frequency bands C1 and C2 which are measured from the top of the curve at the condition of low attenuation to full attenuation at the bottom of the curve. It has been found that an optimum condition exists when the lines 23 and 26 have an electrical length equivalent to about 3A; A, where A stands for the wavelength of the carrier frequency, and that, for lengths greater than the optimum C2 decreases, while for lengths less than the optimum C1 decreases. These changes are indicated by the arrows 27 and 28 and show that the cutoff characteristics of either the upper side band or the lower side band of the filter may be improved at the expense of the cutoff characteristics of the other side band, merely by varying the length of lines 23 and 26. As is well known, the widths C1 and C2 are also affected by the Q value of cavities 22 and 24 and these factors in turn are affected by the tightness of coupling of the cavities tothe remainder of the circuit. The cavities are coupled by extending the center conductors of the lines 23 and 26 into the cavities 22 and 24 respectively to act as small antennas and the tightness of coupling is proportional to the length which the central conductors extend into the cavities. Alternatively, other known types of coupling to the cavities 22 and 24, such as loop coupling, may be employed instead of antenna, or probe, coupling.

Fig. 3 shows a symmetrical filter circuit incorporating the simple circuit of Fig. l. Six magic T hybrid rings are used in the circuit of Fig. 3 in order to achieve the bandwidth required to filter out the lower side-bandk of a television signal operating in the 70() mc. region. For other uses, it might not be necessary to employ more than four of these hybrid rings.

The first hybrid ring 20 comprises arms 5 8. The relative impedances of arms 5-S, as well as the remainder of the circuit, are indicated on the drawing and the lengths of all arms in all of the hybrid rings are approximately although other impedances and other sizes may be substituted as shown in the text referred to above. A source 9 ofsignals is connected by means of a transmission line 1t) to the junction of arms 5 and 6 while a circuit 3 15,usually a'dissipative load,islconnected by means vof a transmission line 29 to the junction of arms 5 and 7. The junctions of arm 8 with arms 6 and 7 are symmetrically connected ftolthe remainder oflthefilterfcircuit'for purposes which will be described more completely hereinafter.

Thejunction of-arms6andSlistconnected bymeans-of a transmission line 31 to the junction of -'arms 132 `and 33 .of afsecond hybridring magic T network 34. Ringll is completed by arms 36 and 37.

.Arst cavity resonator 38lis connectedtothejunction of arms 33 and 36 bya transmission-line 89 while a second cavityresonator 41.s.connected by-means of `atransiriission line 4210 the junction of lines 36 and 37. Liines 39 and 42 correspond to .lines 12 and 1 6 in IFig. l, and, as -described in .connection therewith, are l'preferablycoristructed .with a 'length .of .1i/sk.

The circuitsyrnmetrical to ring S34 fis ring 43 comprising arms li4-4.7 and Aconnected to Vring A20 Y'by a transmission line48 which sis :preferably of the same length las transmission Lline .31; .although the ilength vof rthese two lines is otherwise arbitrary. Cavity resonators 49 and 51 are connected to the ring43 by transmission lines 52 and 53 respectively and arepreferably of the same length as lines 39 `and 42 `respectively.

vRing 34 may :be connected to another ring network .Stif it .-is founddesirable tofernploy the additional =lter ing made possible thereby. Ring :54 `comprises .the four arms 513-59. Cavitiesrl and-.62 are ,connected `to the ring 54 by transmission .lines y63 ,and .64, respectively, which Yfollow ythe Vsame general rule of being approxif -mately /sk long, and ring 54 iis .connected tothe ,ring 34 by means of `transmission line 66.

Symmetrically Yarranged with respect vto ring .154'is a ring 67, comprising arms '68-71, which is connected lto the ring 43 by means of a Vtransmission line 7,2. in order to maintain the desired symmetry line 72 -is preferably of .the same electrical length as line 66. Resonant cavities .'73 and 7 4 are connected to the -ring-67 .byfmeansiof transmission lines 76 and .'77, respectively, which, as before, are preferably symmetrical with 'lines 63 and 64.

An output ring 78 `comprising .arms 79--82 :serves as an output .coupling network .for tthe .ilter. If rings 54 and .6'7 are omitted, ring 78 Ais connected-directly .to rings 34 and 43; otherwise the ring 78 .is connected vby means of symmetrical .transmission lines 83 land -84 Vto the .rings 54 and 67 respectively. A utilization device 86, `similar tothe utilization device 16 .in Fig. 1, 'is rconnected by means f a transmission line :87 to the junction of lines 79 and `82-ofnetwork 78.

Asis described in the text Vreferred to above, magic T networks, whatever zthe form, are characterized by pairs of terminals which .are isolated from `each other .in the sense that anelectrical signal connected to one -of these pairs of terminals produces no output signal at the other end of these terminals. This condition is true of each of the rings in Fig. 3. For instance, under normal conditions where the impedances are preferably matched, the application of asignal from source 9 to the ring 20 produces no direct output .signal to .the dissipative load 15. Instead load .IS receives its .signal from the wave reflected by the cavity resonators connected vto the rings 34 and 43 (as well as by resonators connected :to -rings 54 and 67 if they are used). A

As shown in Fig. 2, the .cavity resonators .of a given hybrid ring are tuned to different frequencies within the band of operation, thereby producing a narrowffrequency band within which the attenuation is very high. -It `is the frequencies within this band which` are reected into the dissipative load 15. Therefore, when an entire double sideband television signal produced :by source 9 is applied to the complete network of Fig. 3, that part of the Ysignal lying between j, and f2 as shown in Fig. 2 is absorbed by the dissipative load. The cavity resonators of rings 34 and 43, if properly and symmetrically adjusted, can be made to resonate so that `this band bttween fl and -f2reovers part of `the 'lower sideband `of the entire television signal and this Apart of the lower sideband will therefore be eliminated from the signal as transmitted to the utilization device 86. In this way the legally required vestigial sideband signal is produced.

The reason for the additional rings 54 and 67 may be shown by referring to Fig. 4 which depicts the steps of tuning the entire -tilter network. In step a none of the `cavity `resonators of Fig. 3 have vbeen adjusted and the entire bandfof :frequencies from the source `9'passes to the utilization device 86. Step b indicates 'the narrow band rellected to ythe load 15 when the most selective of all ofthe cavities 'in Fig 3 is -tunedto its proper resonant point f1. Since a symmetrical condiiton is required, this means that a symmetrically arranged pair of cavity resonators must be tuned to frequency f1. These two symmetrically arranged cavities might be 38 and 49 or they might Ebecavities -41 and 51, etc.

Following the proper tuning-of `this first pair of cavities, ya second "pair -of'cavities -is tuned ito "the frequency f3. As before, the tuning must be symmetrical, and, if it `:be assumed athat 'resonators 38 and 49 are to be tuned to frequencyjl, `-then fitmay be `that-resonators 41 and 51 -will tbe tuned xto `Vfrequency f3. This `second pair of resonators is preferably Vadjusted to Eleave a Apositive peak between the frequencies f, and v12,.

NeXt, another pair -of cavities is ftuned to a frequency f, :between :frequencies f, and ligand adjusted to ysmooth the response, 4:and -iinall-y the fourth xpair Aof cavities is tuned Lto :the frequency T2.

IStep .le `shows a symmetrical l`response of the overall ilter,.but it is frequently `desirable lto obtain -an asymmetrcal response. This may -`Ibe done by increasing the lengths `of ltransmission lines -connected to the proper cavities ias `described in connection with lines 23 and 26 of Figa 1l. For instance, if cavitiesl62 and 74 are tuned-to resonate lat frequency f2, -the lines 64 and'77 may be lpositioned to be :greater than Ethe optimum Vlength of thereby slanting fthe portion 88-of'1'the curve iin step 5 -to the left.

.A second source 89 vis shown in dotted lines :in Fig. 3. -In :the usual rtelevision terminology, source 89 may be the sound :transmitter of `a television station while source "9 may .be the video transmitter. 'Source '89, Vif 'it is connected to the remainder of the circuit, simply supplies an additional signal to `rthe Autilization device A86. Due to the-symmetry of the circuit, the -signal `from source -89 does lnot flow into the ysource V9, nor does the signal from source 9-'ow into :the lsource 89. The vcomplete filter thus serves as Va mixer, or duplexer, -for the two signals.

While this invention has been described in terms of specific embodiments, it will be understood that other modifications -may be made, :all 'of which fall Within the scope or" the following claims.

What =is claimed is: v

Vl. -A symmetrical electrical circuit comprising lfirst, second, third, and fourth magic T networks leach having rst, second, third, 'and founth terminals, the ifirst and second lterminals :of 'each :of said networks lbeing substantially electrically isolated from each other tor electrical signals within apredetermined band Iof frequencies, *a rst source of electrical signals within said -band connected to the rs't terminal of said lfirst network; a utilization circuit connected to the rst terminal of said second network; Va second source of `electrical.signals within said :band connected 'to thesecond 'terminal of said second network; a load -circuit connected 'to the .second terminal of said rst network; a rstpair of connections of equal length linking the third and fourth terminals of said rst network to :the lthird terminals `tif-said third andj'fourth networks, respectively; asecond pairof connections of equal length connecting the third vand ifourth terminals of 'said second network to the fourth terminal -of said third 'and fourth networks, respectively; a first pair of filters tuned to substantially the same first frequency within said band and connected to the first terminal of said third and fourth networks, respectively, by connections which are approximately 2%; of the wavelength of said first frequency; and a second pair of lters tuned to substantially the same second frequency in said band and connected to the sec-' ond terminals of said third and fourth networks, respectively, by connections which are approximately of the wavelength of said second frequency.

2. The circuit of claim 1 in which the 1%; wavelength connections connecting said rst pair of ilters to the first terminals of said third and fourth networks are equally and correspondingly variable in electrical length to adjust the cutoff characteristics of said electrical circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,416,790 Barrow Mar. 4, 1947 2,498,548 Howard Feb. 21, 1950 2,531,419 Fox Nov. 28, 1950 2,532,539 Counter Dec. 5, 1950 2,593,120 Dicke Apr. 15, 1952 2,632,808 Lawson Mar. 24, 1953 

